Modern Concept Science and Environment - 7 43
FACT WITH REASON
Why are handles of metal cutting scissors longer than blades?
Handles of scissors are longer than blades because longer handles multiply the force to cut easily.
ii) Second class lever MEMORY PLUS
The mass added in a wheelbarrow
A lever with the fulcrum and the effort on either is the load, the fulcrum is the wheel,
side of the load is called the second class lever. For and the force is at the end of the
example, nutcracker, lemon squeezer, wheelbarrow, handles where a person lifts it.
bottle opener, etc.
Wheelbarrow Nutcracker Bottle opener
Second class lever
FACT WITH REASON
Second class lever always multiplies the effort applied. Why?
In the second class lever, the effort arm is always greater than the load arm. So, the second
class lever always multiplies the effort applied. A small force can overcome a heavy load in
the second class lever.
c) Third class lever MEMORY PLUS
The longer the lever the higher you
A lever with the load and the fulcrum on either can lift an object.
side of the effort is called the third class lever. For
example, fire tongue, forceps, shovel, a hand with
load on palm etc.
Shovel Fishing rod Fire tongs Broom
Third class lever
FACT WITH REASON
The third class lever is used as a speed multiplier. Why?
In third class lever, the effort distance is always less than the load distance. It cannot be used
as a force multiplier. So the third class lever is used as a speed multiplier. For example, work
becomes faster while throwing the soil with the help of a shovel, etc.
44 Simple Machine
Pulley
The metallic or wooden circular disc having a groove along its rim Pulley
and capable of rotating about an axis passing through its center is
called a pulley. It changes the direction of the applied force. A
common traditional use of pulley is to lift buckets of water from a
well. Nowadays several electrical devices, vehicles, cranes etc. are
installed with pulleys.
Types of Pulley MEMORY PLUS
a) Fixed Pulley The pulley was first invented by
Archimedes.
A pulley which does not move up and down with
load and rotates at a fixed position is called a fixed pulley. For example, the pulley
which is used to withdraw water from a well is a fixed pulley. The load is attached to
one end of a rope which passes over the pulley. The effort is applied at another end.
In the case of a single fixed pulley, the effort distance is equal to the load distance.
The effort applied is more than the load to be lifted. But the force can be applied
downward in a convenient direction.
FACT WITH REASON
Why do we use single fixed pulley even though it cannot multiply force?
We use single fixed pulley even though it does not multiply force because it can change the
direction of force to make our work easier.
It is easy to lift a heavy load with the help of a fixed pulley even without any multiplication of
effort. Why?
By using a fixed pulley, it is possible to apply the effort in a downward direction which makes our
work easy.
Some common uses of a Single Fixed Pulley
1. A single fixed pulley on a fishing rod helps us to catch a fish.
2. Hosting a flag on a flagpole becomes easier with the help of a single fixed pulley.
3. It is used to drop a bucket into the well, collect water, and pull it up again.
b) Movable Pulley
A Single Movable Pulley has a pulley suspended on a rope. One end of the rope is
fixed to a rigid support. Effort is applied at the other end. The load to be lifted is
hung on the pulley. A pulley which rotates as well as moves up and down with load
is called a movable pulley. In case of a single movable pulley, the effort distance is
double the load distance.
Modern Concept Science and Environment - 7 45
Differences between a Single Fixed Pulley and Single Movable Pulley
Single Fixed Pulley Single Movable Pulley
1. Single fixed pulley doesn't move up 1. Single movable pulley moves up and
and down with a load. down with a load.
2. It changes the direction of effort applied. 2. It multiplies the effort applied.
c) Compound Pulley or Block and Tackle
A mixed form of fixed pulley and movable pulley in required numbers is called a
compound pulley. The upper set of pulleys attached to a rigid support is called block
and the lower set of pulleys which carries the load is called tackle.
Effort
Force
Force
Single fixed pulley Single movable pulley Block and tackle system
In cranes, the block and tackle system helps to lift heavy loads and machines.
However, the disadvantage of the block and tackle is that the effort applied travels a
long distance.
FACT WITH REASON
A block and tackle system is used to lift or pull heavy loads. Why?
The block and tackle changes the direction and multiplies force at the same time. The fixed block
changes the direction and movable block multiplies the force. So a block and tackle system is
usually used to lift or pull heavy loads easily.
A block and tackle is also called a compound pulley. Why?
Both fixed block and movable block are there in a block and tackle. The upper fixed part changes
the direction of effort applied and the lower movable part multiplies the effort applied. So, a block
and tackle is also called a compound pulley.
Wheel and Axle
A system of two co-axial cylinders of different diameters
which rotate together is called the wheel and axle. The
cylinder with the larger diameter is called the wheel and the
cylinder with the smaller diameter is called the axle. Wheel
and axle are common in use in our daily life. For example,
Wheel and axle
46 Simple Machine
the door knob, knob of the tap and screw driver are MEMORY PLUS
common. Efforts can be applied from the wheel or
the axle in the wheel and axle. Wheel and axle rotate Wheel and axle is also a type of lever.
The axle is the fulcrum.
together but the effort distance and load distance are
not the same due to the variation in the diameters of the two wheels. Wheel and axle
cannot increase both the force and the speed at the same time. It can work either like
a force or a speed multiplier.
a) Wheel and Axle as a Force Multiplier
When we apply effort from the wheel, in Water tap Steering
wheel and axle, the force gets multiplied. For wheel
example: steering wheel, screw driver and Screw driver
door knob multiply our applied effort. The
larger the diameter of the wheel the more the force multiplies.
FACT WITH REASON
Steering wheel multiplies the effort applied by a driver on it.
When effort is applied from a wheel in a wheel and axle then the effort distance becomes longer
than the load distance. The effort gets multiplied on the axle in this condition. So the steering
wheel multiplies the effort applied by a driver on it.
Steering of a truck is bigger than that of car. Why?
Truck is heavier than car. So, trucks have bigger steering wheel than car to multiply force many times.
b) Wheel and Axle as a Speed Multiplier
When we apply effort from the axle, the speed gets multiplied. For example: LATTAI,
wheels on vehicles, fans, etc. multiply speed.
FACT WITH REASON
Rolling of string becomes faster on a LATTAI, why?
When the load is attached to the wheel and effort is applied from the axle of
a wheel and axle, load distance becomes longer than the effort distance. In
this condition, speed gets multiplied. So rolling of string becomes faster on a
LATTAI.
Wheel and axle can’t increase both the force and the speed at the same time. Why?
We can either apply effort from the wheel or from the axle to get our work done by a wheel and
axle. When the effort is applied form the wheel in a wheel and axle, it multiplies the effort applied.
But, when the effort is applied form its axle, the wheel and axle work like a speed multiplier.
Modern Concept Science and Environment - 7 47
Inclined Plane
A plane lying along a slope with a certain angle on h l
the earth’s surface is called an inclined plane. For
example, an uphill road, ladder, children’s slide, Inclined Plane
etc. Heavy loads are loaded in trucks by applying a
small force with the help of an inclined plane.
Conditions in which work becomes easier or harder in using an inclined plane
When the length of an inclined plane is longer than its height, the work becomes
easier. But, if the length of an inclined plane is shorter, then more effort is required
and the work becomes difficult.
Applications of Inclined Planes
1. Heavy loads are loaded in trucks with the help of inclined planes.
2. Inclined planes are made in hospitals so that wheelchairs can be easily pushed.
3. Hill roads have slopes so that a vehicle can move up easily.
FACT WITH REASON
Vehicles need less effort to carry heavy load on the roads going uphill. Why?
Due to the winding of roads on the hills, their inclination increases slightly. This makes effort
distance longer and the effort multiplies. So, the vehicles need less effort to carry heavy loads on
roads going uphill.
Heavy loads are placed in trucks with the help of inclined planes.
The slanted surface of an inclined plane multiplies our effort applied. Heavy loads can be placed
in a truck by applying a small force with the help of an inclined plane.
Screw
A nail with spiral inclined threads on it is called a Jack screw Screw nails
screw. A screw has an inclined plane in the form
of threads. So it is also called a modified form of
inclined planes. For example, screw nail, Jack screw,
wood screws etc. The distance between the two screw
threads is called a pitch.
Nuts and bolts are used for tightening the metallic sheet and rod, wooden plunk,
etc. Jack screw is used to lift heavy loads, especially vehicles. A car or any other load
which is to be raised is placed on the top of the jack screw. The jack screw is turned by
a horizontal bar connected to the lever. As the lever completes one rotation, the screw
also rotates once and the load moves up.
48 Simple Machine
FACT WITH REASON
Screw is also called a modified form of inclined planes. Why?
A screw has an inclined plane in the form of spiral threads. So, it is also called a
modified form of inclined plane.
Wedge
We use a number of sharp ended tools in our daily life. For example, axe, KHUKURI,
knife, blade etc. They are called wedge. Thus, the simple machine which has one end
thin and another end flat is called a wedge. In a wedge, effort is applied from the flat
end and output work is obtained from the sharp end. There are two slanted surfaces
on a wedge which help in multiplying the effort applied.
Condition of multiplication of effort in a wedge
The longer the sharp end in the case of a wedge the easier is work done against a
heavier load.
Axe Khukuri Knife
STEPS EXERCISE
STEP 1
1. Tick () the correct statement and cross () the incorrect one.
a) Bicycle is an example of a complex machine.
b) In the case of a lever, the distance of load from the fulcrum is called the
load distance.
c) In a simple machine, if the load distance is longer than the effort distance,
then the effort gets multiplied.
d) A second class lever always multiplies the applied effort.
e) A steering wheel of a car is an example of a pulley.
f) A movable pulley multiplies effort to make our work easier.
g) The thread of a screw is an inclined plane wrapped round the screw.
2. Fill in the blanks with appropriate words.
a) A complex machine consists of several ……. machines working together.
b) In the case of a lever, the distance of effort from the fulcrum is called …… .
c) It becomes easy to do work when the effort distance is greater than …….
in a simple machine.
Modern Concept Science and Environment - 7 49
d) Scissors is a …… class lever.
e) The point about which lever rotates is called…… .
f) …….....pulley changes direction of force to make work easier.
g) A single movable pulley makes our work easier by …….. effort.
STEP 2
3. Answer in one word
a) Which type of simple machine is an axe?
b) Which kind of simple machine is door handle?
c) Which simple machine has a groove on its circumference and runs with
the help of rope?
d) Give an example of a simple machine which multiplies effort.
e) What is the term for a simple machine with threads in the form of an
inclined plane?
4. Differentiate between
a) second class lever and third class lever
b) fixed pulley and movable pulley
c) effort distance and load distance
d) screw and inclined plane
5. Give reason
a) Use of single fixed pulley makes it easier to lift load.
b) It is easier to overturn a heavy stone with the help of a crowbar.
c) Force gets multiplied on applying effort from the wheel in a wheel and axle.
d) A pulley has a groove cut along its rim.
e) A sharp knife makes it easier to cut than a blunt knife.
f) Winding roads are made on hills.
6. Answer the questions with the help of the given figure
a) Write the name and use of simple machines shown in the figures below.
i) ii) iii) iv)
b) Look on the figure shown and write the name of B
parts A, B, C and D. D A
C
50 Simple Machine
STEP 3
7. Answer the following questions.
a) What is a simple machine?
b) Give four reasons for using simple machines.
c) How many types of simple machines are there? Write their names.
d) What is lever? Write its types.
e) Define.
i) fulcrum ii) effort iii) effort distance
iv) load distance v) first class lever vi) second class lever
vii) third class lever viii) fixed pulley ix) movable pulley
x) combined pulley
f) What is a pulley? For which purpose is it used?
g) How many types of pulleys are there? Write their names.
h) What is a wheel and axle? Give examples.
i) What is an inclined plane? Give examples.
j) How will you load a heavy drum into a truck?
k) What is a screw? Give examples.
l) What is a wedge? Give examples.
m) Write the condition for following:
i) to multiply effort with the help of a simple machine.
ii) to complete our work faster with the help of a simple machine
iii) to multiply effort with the help of a wheel and axle.
iv) to make work easier by using an inclined plane.
v) to make our work easier by using a wedge.
8. Classify the given simple machines.
broom, spoon, knife, axe, scissors, screw nail, jack screw, DHIKI, ladder,
wheelbarrow, shovel, bicycle paddle
9. Draw the diagram
a) to show the position of fulcrum, load, effort, effort arm, and load arm in a
crowbar when it is used to lift a heavy load.
b) to show a single fixed pulley, single movable pulley, and compound pulley.
UNIT Estimated teaching periods TheoMryoderPnraCcotniccaepl t Science and Environment - 7 51
31
4
Pressure
Syllabus issued by CDC Pressure
Introduction to pressure
Measurement of pressure
Differences between force and pressure
Application of pressure in our daily life
LEARNING OBJECTIVES
At the end of this unit, students will be able to:
introduce pressure and explain the application of pressure in our daily life with
examples.
solve some simple numerical problems related to pressure.
Key terms and terminologies of the unit
1. Thrust: A force acting normally on a surface is
called the thrust.
2. Pressure: The force acting perpendicularly per unit
area is called the pressure.
3. One pascal pressure: The pressure exerted by 1N force normally
on an area of 1m2 is called 1pascal
pressure.
4. Relationship between pressure and force: Pressure increases with the increase in
force and pressure decreases with the
decrease in force.
5. Relationship between pressure and area: Pressure decreases with the increase
in area and pressure increase with the
decrease in area.
4.1 Introduction
In our daily life, we realize pressure as one of the most important concepts. If a knife
is sharp, the area of its contact with the object to cut becomes small, so we need to
apply less force to cut vegetables, fruits, etc. The injection needle is also made very
sharp at its tip. It needs less force to push such a sharp needle through the skin. The
52 Pressure
force exerted per unit area is more in the case of the sharp objects and less in the case
of the blunt ones. Therefore, the force acting perpendicularly per unit area is called
the pressure. Pressure is a scalar quantity.
A force acting normally on a surface is called thrust. It is a vector quantity. Pressure is
also defined as the thrust per unit area.
FACT WITH REASON
Pressure is a scalar quantity. Why?
Pressure is a scalar quantity because it has only magnitude without a specific direction.
Relationship among force, area and pressure MEMORY PLUS
The same force can produce different pressure Weight Weight
depending on the area over which it acts. For 100N 200N
example, when we stand on only one foot, more
pressure is exerted on the ground. But if we stand A = 0.1 m2 A = 0.1 m2
on both feet, less pressure is exerted on the ground. P = 1000 pa P = 2000 pa
Less weight More weight
Pressure depends on the following two factors less pressure more pressure
a) Force applied : Pressure increases with the Weight Weight
increase in the force applied and vice versa. 100N 100N
b) Surface area : Pressure decreases with the A = 0.1 m2 A = 0.01 m2
increase in the surface area and vice versa. P = 1000 pa P = 10,000 pa
More area less Less area more
Calculation of pressure pressure pressure
Pressure depends on the applied force and area. If ‘F’ be
the applied force and ‘A’ be the area then,
Pressure (P) = Force (F)
Area (A)
SI Unit of pressure MEMORY PLUS
The SI unit of pressure is N/m2 or pascal (Pa).
1. Pressure and force both are
1 pascal pressure: derived quantity as they depend
According to formula, upon two or more fundamental
quantities.
Pressure = Force 2. In honour of scientist Blaise
Area Pascal, the SI unit of pressure is
given pascal(Pa).
or, 1 Pa = 1 N
1 m2
The pressure exerted by 1N force acting normally upon an area of 1m2 is called 1
pascal (1Pa).
Modern Concept Science and Environment - 7 53
Solved Numerical 4.1
Find the pressure exerted by the box of 400N, which occupies an area of 2m2 on the
ground.
Solution: Given,
Here, weight of the box (F) = 400N
Area occupied by the box (A) = 2m2
According to the formula,
Pressure = Force = 400 = 200 N/m2 or Pa
Area 2
The pressure exerted by the box on the ground is 200 Pa.
Solved Numerical 4.2
An elephant weighing 40,000N stands on one foot of area 1000 cm2. Find the pressure
exerted by the elephant on the ground.
Solution: Given, MEMORY PLUS
Force (F) = 40,000 N 1. Gas and liquid both are considered
Area (A) = 1000 cm2 = 1000 = 1 = 0.1 m2 as fluids. Fluids exert pressure in
all directions.
100 × 100 10
According to the formula, 2. Manometer is an instrument used
Pressure = F = 40000 = 4,00,000 Pa to measure the gas pressure.
A 0.1 3. Barometer is an instrument used to
Solved Numerical 4.3 measure the atmospheric pressure.
A girl weights 400N and stands on one stiletto heel, which occupies an area of 1cm2.
Find the pressure exerted by her on the ground.
Solution: Given
Force (F) = 400 N
Area (A) = 1 cm2 = 1 = 1 = 0.0001 m2
100 × 100 10000
According to the formula,
Pressure = F = 400 = 40,00,000 Pa
A 0.0001
54 Pressure
Differences between Force and Pressure Pressure
Force Pressure is the force acting normally
1. Force is a push or pull which changes 1. per unit area.
or tries to change the state of a body. It is calculated by the formula, P = F .
2. It is calculated by the formula, F = m a 2. A
3. SI Unit of force is newton (N). 3. SI Unit of pressure is N/m2 or Pascal
(Pa).
Explanation of Some Pressure Based Everyday Observations
A school bag has wide straps.
Wide straps of the bag fall over a larger area of the shoulder of a child.
It produces less pressure on the shoulder. So, with the help of the wide
straps, the child feels it comfortable to carry the heavy school bag.
Studs are made on the football boot.
The studs on a football boot have only a small area of contact with
the ground. This results in a high pressure, which gives extra grip,
and football players do not slip while running on the ground.
The tip of a needle is made sharp.
The sharp tip of a needle covers very small area of the surface to be
penetrated. Due to this it creates a large pressure and penetration of a
surface becomes easy.
A sharp knife cuts better than a blunt knife.
A sharp knife has a very thin edge to its blade. Due to this the force of
our hand falls over a very small area of the object. This creates a large
pressure and the knife helps to cut the objects easily.
It is easy to walk on soft sand if we have flat shoes
Flat shoes occupy more area on the sand and they exert less pressure on the soft sand.
Due to this, our feet do not go down deep in sand and it becomes easy to walk on the
soft sand.
A camel walks on sand with ease but it is difficult for a man to walk on the sand
despite the fact that a camel is much heavier than a man
Camel hooves cover a large area. The weight of the camel falls on a large area of sand.
Due to this, the pressure created on the sand becomes less and camel’s feet do not
go deep in sand. On the other hand, the weight of the man exerts pressure on sand
through the feet, within a small area. This results in more pressure on the sand and it
becomes difficult for the man to walk on it.
Modern Concept Science and Environment - 7 55
Soil is pressed less by flat slippers and more by heel sandals
Flat slippers occupy more area than a heel sandal. Due to this the flat
slippers exert less pressure and heel shoes exert more pressure.
Tractors have broad tyres
The wide tyres of a tractor spread its weight over a large area of the
ground. Due to this, there is less pressure on the ground and the
tractor tyres do not dig too dip. If the tyres are made thin, they occupy
less area and dig in more due to more pressure.
More wheels are used in pairs on heavy trucks
Heavy trucks exert more pressure on the ground. The ground-
wheel contact area increases by using more wheels in pairs on heavy
trucks. Due to this the pressure exerted on the ground decreases and
the trucks can carry heavy loads easily.
The railway tracks are laid on large sized wooden, iron or cement sleepers
When large sized wooden, iron or cement sleepers are kept below
the iron rails of a railway track, the thrust due to heavy weight of
the train spreads over a large area. Due to this, the railway line
does not sink into the ground.
FACT WITH REASON
A wide steel belt is provided over the wheels of army tanks. Why?
Pressure exerted on the ground is inversely proportional with the area occupied by a body. Wide
steel belts over the wheels of the army tanks occupy more surface area and exerts less pressure.
Foundations of buildings are made wider. Why?
There is wide foundation of buildings in a larger surface area inside soil. It reduces the pressure on
the ground. So the foundations bear the pressure due to heavy load of the building.
Pressure on the ground is more when a man is walking than when he is standing
When a man is standing, then both the feet are in contact with ground and surface area in contact
with the ground increases. Due to this the pressure exerted on the ground decreases. On the
other hand, while walking, the weight of a man falls on less surface area of left and right legs
alternatively. This results in more pressure on the ground.
STEPS EXERCISE
STEP 1
1. Tick () the correct statement and cross () the incorrect one.
a) Pressure is inversely proportional to the force.
b) Pressure is measured in N/m2.
56 Pressure
c) Heel shoes dig in more than slippers do.
d) Skiers use long flat skis to slide over snow.
e) Standing on a leg increases pressure on the ground.
f) Front wheels of a tractor are flat.
g) Animals like the camel can easily walk fast in the desert.
2. Fill in the blanks with appropriate words
a) Force acting on a unit area is called ……..
b) The SI Unit of pressure is …… .
c) Pressure depends upon …… and …… .
d) Pressure increases when there is increase in…… .
e) Pressure ……. when the surface area decreases.
f) Tip of a needle is made sharp to increase …… .
g) Pressure is a ……. quantity.
STEP 2
3. Answer in one word.
a) What is the term for the force acting perpendicularly per unit area?
b) How many Newton of force is applied per unit area to exert a pressure of
1Pa?
c) Write one out of the two factors which affects the pressure.
d) Does the pressure exerted by a shoe on ground increase or decrease by
making studs on its sole?
e) Does the pressure exerted by a needle increase or decrease by making its
one end pointed?
4. Differentiate between
a) Force and pressure
5. Give reason
a) School bags are provided with wide straps to carry them.
b) Wooden sleepers are laid below the railway line.
c) A flat slipper digs in less than a heel shoe while walking on the soft sand.
d) Back wheels of a tractor are made flatter.
e) A number of back wheels are used on heavy trucks.
Modern Concept Science and Environment - 7 57
f) Football players wear shoes with studs on them. i ii
g) One end of a needle is made pointed.
h) Foundation of a building is made wider than its walls.
6. Answer the questions with the help of the given figures.
a) In which case is the pressure exerted on the ground more?
b) In which case, in the given figures, is the pressure i ii
exerted on the palm more. Write with reasons.
STEP 3
7. Answer the following questions.
a) What is pressure? Write its SI Unit.
b) How is pressure calculated?
c) Define 1Pa of pressure.
d) Write the relationship of pressure with the applied force and area.
e) How would pressure change if
i. the area is doubled by keeping the force constant
ii. the force is doubled by keeping the area constant.
8. Numerical problems
a) A box of 400N covers an area of 2m2. Calculate the pressure exerted by the
box on the ground. [Ans: 200 Pa]
b) A man of 450N covers an area of 0.015m2 while standing on his feet.
Calculate the pressure exerted by him on the ground. [Ans: 30,000 Pa]
c) A wooden box of dimensions 50cm × 30cm × 20cm is kept on the ground.
The mass of the box is 30kg. Find the pressure exerted by the box on the
ground. [Ans: 2000 Pa]
9. Draw the diagram
a) to show the relationship between force and pressure when the area of
contact is kept constant.
b) to show the relationship between area of contact and pressure when the
force applied is kept constant.
58 W ork , E nEsetrimgayteatenadchPinog wpeerirods Theory Practical
UNIT 3 0
5 Work, Energy and Power
Syllabus issued by CDC Crane
Introduction to energy
Types of energy
Introduction to work
Types of work
Power
LEARNING OBJECTIVES
At the end of this unit, students will be able to:
introduce energy, work and power.
explain the types of energy.
explain the types of work.
Key terms and terminologies of the unit
1. Work: The work is said to be done when a body moves in the direction
of force applied.
2. One joule work: One joule of work is done when a force of 1N displaces an
object through a distance of one meter in the direction of force
applied.
3. Work against friction: The work done in pushing or pulling a load by applying force
against friction is called work done against friction.
4. Work against gravity: The work done in lifting weights by applying force against
gravity is called the work done against gravity.
5. Energy: The capacity of a body to do work is called energy.
6. Mechanical energy: The energy of an object due to its motion or position is called
the mechanical energy.
7. Kinetic energy: The energy possessed by a body in motion is called the kinetic
energy.
8. Potential energy: The energy possessed by a body due to its position or
deformation is called the potential energy.
Modern Concept Science and Environment - 7 59
9. Power: Power is the rate of doing work.
10. One watt power:
Power is said to be one watt when one joule of work is done in
one second.
5.1 Introduction
Normally, the word work is used to describe any activity which has been carried out
in our day to day life. In physics, work is related to the force applied on a body and
the corresponding displacement. Naturally, water in a river flows down the slope,
meteors hit the earth’s surface and animals perform several basic life processes. Such
activities constitute work. There is a need of energy to do these types of work.
In the case of machines, they need different sources of energy like electricity, petrol,
diesel etc. Different machines have different capacities to do work at different rate.
This rate of work done gives power of the machine. In this unit we will learn about,
work, energy and power.
FACT WITH REASON
Guarding by standing in front of a gate is not called work in physics, why?
In physics, work is related to the force applied on a body and the corresponding displacement.
Guarding by standing in front of a gate is not called work in physics because a standing
person does not cover a distance from its position of rest.
5.2 Work
The meaning of work in our daily life is quite different than that in physics. For
example, a watchman standing in front of a gate, a person standing at a place by
holding some load on his head, a dog tied to a gate, etc. are not doing any work in the
language of physics.
In physics, the word ‘work’ is used only in cases where there is a force acting on a
body and the body moves by a certain distance. Work is done whenever a force acting
on a body results in a displacement in the direction of the force. For example,
a) When a ball is thrown horizontally, the ball covers some distance along the
direction of the force applied and work is done on the ball.
b) When we pull a box on the ground, the box covers some distance along the
direction of the force applied and work is done on the box.
The work is said to be done when a body moves in the direction of force applied.
Mathematically, work is the product of force and displacement in the direction of
force. It can be expressed by the given formula.
60 Work, Energy and Power
i.e. W = F × s s F
Where, F = Force applied in the direction of the displacement
s = Displacement in the direction of the force θ = 0°
Work
FACT WITH REASON
Work is a scalar quantity.
Although work results from the product of two vector quantities (force and displacement), it is only
the magnitude that matters in the description of work, not the direction. So work is a scalar quantity.
Factors Affecting Work
a) Magnitude of the Applied Force:
Work done is directly proportional to the force applied. In the case of a work to be
done, more value of the force applied results in a greater amount of work done.
b) Magnitude of Displacement:
Work done is directly proportional to the displacement. The greater the displacement,
the greater is the work.
Units of Work MEMORY PLUS
SI Unit of work is joule (J). CGS unit of work is erg. 1. The unit of work and energy i.e
Relationship between joule and erg Joule is named after the British
Since, W = F × s physicist James Prescott Joule.
2. The word erg is derived from the
Greek word for work; ergon.
1J=1N×1m
1 J = 105 dyne × 100 cm
∴ 1 J = 107 erg
Other Units of Work
1 kilo joule (kJ) = 103 joule (J) 1 mega joule (MJ) = 106 joule (J)
1 giga joule (GJ) = 109 joule (J)
FACT WITH REASON
Mr. Manoj is pushing a wall but the wall is not moving. Can it be called a work? Give reason.
No, it cannot be called a work in language of physics because distance is not being covered by
the wall.
Even when there is no tangible work done, we get tired.
If we push against a wall for a long time, we will expend calories and get tired. This is because our
muscles need to continuously produce and consume energy to maintain a force, even when there
is no net displacement. The energy produced is dissipated as heat.
Modern Concept Science and Environment - 7 61
Solved Numerical 5.1
The mass of a car along with the driver is 1000kg and it accelerates with 6m/s2 within
the distance of 500m. Calculate the work done by the engine of the car.
Solution: Given
Here, displacement of car (s) = 500m
Mass of the car (m) = 1000kg
Acceleration (a) = 6m/s2
Now, Work done (W) = Force × displacement = ma × s =1000×6×500 J
or, W = 3000000 J = 3000 kJ 1 J of work
1N
1 joule work
One joule of work is done when a force of 1N displaces an
object through a distance of one meter in the same direction
of the force.
Mathematically, 1m
W=F×s
1 J work
1J=1N×1m MEMORY PLUS
Types of Work A force does no work when the
displacement is zero.
Work Done against Friction
The resistive force between two surfaces in a relative Applied force Friction force
motion is called a frictional force. The work done in
pushing or pulling a load by applying force against Work against friction
friction is called work done against friction. For
example, pushing a book on a table, pulling a box on
the ground, etc.
If ‘F’ is the force of friction and ‘s’ is the displacement of a body then the work done
against friction is given by W= F × s
FACT WITH REASON
Pushing a book is work against friction. Why?
Pushing a book is work against friction because book moves in the opposite direction of the
frictional force.
62 Work, Energy and Power
Solved Numerical 5.2
A box is pushed on the ground for a horizontal distance of 3m and the frictional
force experienced by the box is 100N. Calculate the work done to overcome friction.
Solution: Given
Frictional force (Ff) = 100N
Displacement (s) = 3m
Now, Work done to overcome friction (Wf) = Ff × s = 100 × 3 = 300J
Work Done against Gravity
Objects are pulled towards the center of the earth by gravity. Work against gravity
In order to lift an object, an upward force equal to the weight
must to be applied on the object. The work done in lifting
weights by applying force against gravity is called work done
against gravity. For example, throwing a ball up, lifting a
load, climbing stairs, etc.
The work done against gravity is equal to the product MEMORY PLUS
of the weight of the body and the vertical distance
through which the body is raised. Total work done = work done
overcoming friction + work done
If a mass ‘m’ is lifted to a height ‘h’ then the work against gravity.
done against gravity is given by
Work done against gravity (W) = Force of gravity × displacement
or, W = weight × height
∴ W = mgh
Solved Numerical 5.3
Calculate the work done by a cat of 5kg when it jumps to a height of 4m.
Solution: Given
Mass of the cat (m) = 5kg
Displacement (s) = 4m.
Now, the work done against gravity (W) = mgh = 5× 9.8× 4= 196 J
The work done by the cat is 196J.
FACT WITH REASON
Climbing ladder is a work against gravity. Why?
Climbing a ladder is work against gravity because we move opposite to the direction of
gravity while climbing a ladder.
Modern Concept Science and Environment - 7 63
5.3 Energy
Survival of living beings is impossible without energy. We have the capacity to walk
or to do work due to the energy in our body. This energy comes from the food we eat.
In a machine, there is fuel, which supplies energy for its working. No work can be
done without energy. The capacity of a body to do work is called energy.
Units of Energy
SI unit of energy is joule (J) and its cgs unit is erg.
FACT WITH REASON
The unit of energy is the same as that of work, i.e. joule (J). Why?
Energy is the capacity to do work. It is equal to the amount of work the body can perform. For
example, 1J of energy is the capacity to do 1J work.
Forms of energy
a) Mechanical Energy
The energy of an object due to its motion or position is called the mechanical energy.
A bird flying at a certain height has the mechanical energy. Kinetic energy and
potential energy are the two types of mechanical energy.
i) Kinetic Energy: Moving objects can do work. For Kinetic energy
example, blowing air can rotate blades of a windmill,
flowing water can rotate the turbines, etc. The energy
possessed by a body in motion is called the kinetic
energy. A rolling football, a falling apple, a running
athlete, a moving vehicle, etc. possess the kinetic energy.
The formula to calculate kinetic energy is:
K.E. = 1 mv2
2
Solved Numerical 5.4
If a bullet having a mass of 150g is shot at a muzzle velocity of 960m/s, calculate the
kinetic energy of the bullet?
Solution: Given
Mass of the bullet (m) = 150g = 150 = 0.15 kg
1000
Muzzle velocity (v) = 960m/s
Now, kinetic energy of the bullet is given by
K.E. = 1 mv2 = 1 × 0.15 × (960)2 = 69,120 J
22
64 Work, Energy and Power
Factors Affecting Kinetic Energy MEMORY PLUS
a) Mass
The faster an object moves, the
If the mass of a body is doubled, its kinetic energy more kinetic energy it has.
also gets doubled and if the mass of the body is
halved, its kinetic energy also gets halved.
b) Velocity
If the velocity of a body is doubled, its kinetic energy becomes four times and if the
velocity of a body is halved, then the kinetic energy gets reduced to one-fourth.
FACT WITH REASON
Kinetic energy of a moving object increases if velocity is increased. Why?
Kinetic energy of a moving object increases if velocity increases because kinetic energy is
directly proportional to the square of velocity.
ii) Potential energy
Potential energy is a stored energy. The energy possessed by a body
due to its position or deformation is called the potential energy. For h
example, a compressed spring, stretched catapult and water in a
dam possess the potential energy.
Ground
When a body is lifted vertically upward against the gravitational Potential energy
force, some work is done on the body. Such a work done on the body to raise it is
stored in it in the form of gravitational potential energy.
Let a body of mass ‘m’ be placed at a height ‘h’ above MEMORY PLUS
the ground. Then, the potential energy of the body
= Work done against gravity in raising the body Although potential energy is stored,
through a height of ‘h’. it has the potential to be used
later. So, Potential energy is called
∴ Potential energy (P) = weight × h = mgh “potential” .
FACT WITH REASON
A stone lying on the ground has no potential energy. Why?
A stone lying on the ground has no potential energy because potential energy is dependent
upon height but its height from the ground is zero.
Potential energy, ‘mgh’ is called the gravitational potential energy. Why?
The potential energy, ‘mgh’ is due to work done against gravity. Hence, it is called a
gravitational potential energy.
Modern Concept Science and Environment - 7 65
Solved Numerical 5.5
Calculate the potential energy of a stone of mass 10kg placed at a height of 4m
above the ground. (g = 9.8 m/s2)
Solution: Given,
Mass of the stone (m) = 10 kg
Height of the stone (h) = 4 m
Acceleration due to gravity (g) = 9.8 m/s2
According to the formula, gravitational potential energy = mgh
or, the potential energy = 10 × 9.8 × 4 = 392 J
Differences between Kinetic Energy and Potential Energy
Kinetic Energy Potential Energy
1. Kinetic Energy is the energy 1. Potential Energy is the energy
possessed by virtue of the motion of possessed by virtue of the position
the body. or configuration of a body.
2. The greater the velocity, the greater is 2. The greater the height, the greater is
the kinetic energy. the potential energy.
3. Kinetic energy is given by 3. Potential energy is given by P = m g h
1 Where, 'm' is the mass of the body, 'g'
K.E. = 2 mv2 Where, 'm' is the mass is the acceleration due to gravity, 'h'
of the body, 'v' is the velocity of the is the height from the earth’s surface.
body.
Other Forms of Energy
i) Heat Energy
Heat is a form of energy which produces sensation of Warming on an electric heater
warmth in us. It is the sum of the kinetic energy of all the
particles in a substance. Heat always flows from a body at MEMORY PLUS
a high temperature to a body at a low temperature. The
heat energy can be converted into other forms of energy. The sun gives us enormous
The heat produced by burning of petrol in an automobile amount of heat and light. The
engine provides the energy to run the vehicles. energy is produced in the
sun due to nuclear reaction.
FACT WITH REASON
Heat is called energy. Why?
Heat is called energy because it can do work and it changes into other forms of energy.
66 Work, Energy and Power
ii) Light Energy
Light is a form of energy which produces sensation of vision in us. It can easily be
converted into electrical energy by the use of photoelectric cells. Green plants directly
use light energy to prepare food during photosynthesis. Light energy causes chemical
change to take place. When the light energy falls on a photographic film, the images
are recorded on the photographic film due to a chemical change.
iii) Sound Energy Loud speaker
Sound is a form of energy which is produced by the vibrating bodies.
Our eardrum vibrates when the sound produced by a vibrating
body falls on it and we hear the sound. We can experience it as a
form of energy. When a stone hits the floor, a part of its mechanical
energy gets converted into the sound energy and heat energy.
FACT WITH REASON
How are we able to speak?
We are able to speak because vibration of vocal cord produces sound from our mouth.
iv) Electrical Energy
Electrical energy is produced by the transfer of electrons from an insulator and flow of
electrons through a conductor. We obtain it from different sources like, dry cell, solar
panels, etc. The electrical energy can be used to run a number of electrical devices
like fan, bulb, washing machine, etc. It is possible to convert the electrical energy into
other forms of energy easily.
FACT WITH REASON
Electricity is called energy. Why?
Electricity is called energy because it does work when we use an electric device and it can be
changed into other forms of energy such as heat, light, etc.
v) Magnetic Energy
A magnet attracts magnetic substances. The energy possessed by a
magnet is called the magnetic energy. The magnetic energy is used
to make an electromagnet, which is widely used in electrical devices.
It is also used to separate magnetic substances from a mixture.
vi) Chemical Energy Magnetic energy
The energy produced due to chemical reactions is MEMORY PLUS
called chemical energy. Petrol, coal, kerosene, etc.
have energy stored in the chemical form. Green Cooking is a good example of a
plants prepare food by photosynthesis. Energy from chemical reaction that releases
such food gets released during respiration. chemical energy.
Modern Concept Science and Environment - 7 67
vii) Nuclear Energy
The nucleus of an atom MEMORY PLUS
has energy stored in it.
Such energy gets released The first nuclear bomb was used by
USA in Japan in 1945 AD.
during the nuclear reaction. It occurs in the stars or atomic
explosions. The energy liberated during nuclear reactions Nuclear energy
(nuclear fusion and nuclear fission) is called the nuclear
energy.
FACT WITH REASON
How is sun able to give us large amount of heat and light energy?
Sun is able to give us large amount of heat and light energy due to nuclear fusion of hydrogen
atoms into helium atoms.
5.4 Power
The time required to complete a work varies as per the worker or the machine. To
know who is a more efficient worker, we find out the value of work done per unit
time. If a machine completes a work faster than another machine, then it has more
power. So, power is the rate of doing work. It is also defined as the rate at which
energy is transferred. Mathematically,
Power (P) = Work done MEMORY PLUS
Time taken
Power is a scalar quantity.
Units of Power
The SI unit of power is J/s or watt (W).
Unit Symbol Equivalence in watt (W)
Horse power h.p. 1 h.p.= 746 W
Kilowatt kW 1kW = 1000W
Megawatt MW 1MW = 106 W
1 watt Power
Power (P) = Work done
Time taken
1W=1J
1s
Thus, power is said to be one watt when one joule of work is done in one second.
68 Work, Energy and Power
Meaning of Power of Electrical Appliances
a) 60W bulb : It means, the bulb consumes 60J of electrical energy to convert it into
heat and light energy in one second.
b) 150W fan : It means, the fan consumes 150J of electrical energy to convert it into
the mechanical energy in one second.
Solved Numerical 5.6
A constant force of 200N pulls a crate for 10m along a level floor in 10s. What is the
power used?
Solution: Given
The force applied (F) = 200N
Displacement (s) = 10m
Time taken (t) = 10s
According to the formula,
Power (P) = Work done (W) = F × s = 200× 10 = 200 W
Time taken (t) t 10
The power used is 200W
Factors Affecting Power
a) Work done (W) b) Time taken (t)
If the time taken to complete the work is less, then the power is more and vice versa.
Differences between Work and Power
Work Power
1. Work is said to be done when the force 1. Power is the rate of doing work.
acting on a body causes displacement
in the direction of the force.
2. Work = Force × displacement 2. Power = Work done
Time taken
3. The SI Unit of work is joule (J). 3. The SI Unit of power is watt (W)
STEPS EXERCISE
STEP 1
1. Tick () the correct statement and cross () the incorrect one.
a) Work done is given by, Work = Force × displacement.
b) Power is measured in J/s.
c) The SI Unit of energy is Joule.
Modern Concept Science and Environment - 7 69
d) A flowing river possesses the potential energy.
e) Water stored at a height in a dam possesses the potential energy.
f) During digestion of food, the chemical energy changes into nuclear energy.
g) The time taken to complete a work increases then power becomes more.
2. Fill in the blanks with appropriate words.
a) The SI Unit of work is …… .
b) Work depends upon …… and …… .
c) Fuels have …… stored in them.
d) Power is given by, P = ................
Time taken
e) 1 horse power (h.p.) = ……. watt.
f) A stretched rubber possess …… energy.
g) Strong wind possess ……. energy.
STEP 2
3. Answer in one word
a) Which type of energy gets released by a chemical reaction?
b) Which quantity does the horse power measure?
c) Which energy is used by plants to prepare their food?
d) What is the term for the energy produced due to kinetic energy of
molecules in a body?
e) What is the term for the energy produced by the vibration of a body?
4. Differentiate between:
a) Work against gravity and work against friction
b) Potential energy and kinetic energy
5. Give reason:
a) No work is done when a man pushes against a wall.
b) Food possesses chemical energy.
6. Answer the questions with the help of the given figure
a) Write the type of work shown in the given figures.
Applied force Friction force
i) ii)
70 Work, Energy and Power
b) Which type of energy is possessed by the following figures.
i) ii) iii) iv)
STEP 3
7. Answer the following questions.
a) When is work said to be done?
b) Is it possible that a force is acting on a body but still the work is zero?
Explain your answer with one example from our daily life.
c) What is energy? Write any two sources of energy.
d) Write about any two types of energy, with examples.
e) What is a mechanical energy? Write about its types with examples.
f) Define
i) chemical energy ii) heat energy iii) light energy
iv) sound energy v) electrical energy vi) magnetic energy
vii) nuclear energy
g) Write any two uses of electrical energy.
h) Write any two uses of magnetic energy.
i) What is work? Write its formula and SI Unit.
j) What is power? Write its formula and SI Unit.
k) Define one watt of power.
8. Numerical problems
a) A girl applies a force of 10N to move a box over a 5m distance on a
horizontal surface. Find the work done. Which type of work is it? [Ans: 50 J]
b) A boy lifts 20N bag vertically up through a distance of 1m. Find the work
done. Which type of work is it? [Ans: 20 J]
c) A boy does 20J of work in 4s. Find his power. [Ans: 5 W]
d) A pump lifts 2000N of water to a water tank placed at a height of 25m in
20s. Find its power. [Ans: 2500 W]
9. Draw the diagram
a) to show the work done against gravity and write the expression to calculate it.
b) to show the work done against friction and write the expression to calculate it.
UNIT Estimate teaching periods TheoMryoderPnraCcotniccaelpt Science and Environment - 7 71
41
6
Heat
Syllabus issued by CDC Electric heater
Introduction to heat
Transmission of heat: Conduction, Convection and Radiation
Structure and utility of thermos flask
Temperature and structure of thermometer
LEARNING OBJECTIVES
At the end of this unit, students will be able to:
demonstrate various methods of transmission of heat and explain them.
introduce temperature.
explain the structure of thermometer and measure temperature by using it.
Key terms and terminologies of the unit
1. Heat: Heat is a form of energy which gives us sensation of the
warmth. It is the sum of the kinetic energy of all the
molecules in a substance.
2. Transmission of heat: The process of transfer of heat from one place to another is
called transmission of heat.
3. Conduction: The process of transfer of heat in solids from one molecule
to the neighboring molecule without their actual movement
is called conduction.
4. Conductor of heat: Materials which allow heat to pass through them are called
conductors of heat.
5. Good conductor of heat: The materials which allow heat to flow through them easily
are called good conductors of heat.
6. Insulator of heat: Materials which do not allow heat to pass through them
easily are called insulators of heat.
7. Convection: The process of transfer of heat by actual movement of
molecules in liquid and gas is called convection.
8. Sea breeze: The cold air above the sea that blows towards the land
during the day is called sea breeze.
72 Heat
9. Land breeze: The cold air above the land that blows towards the sea
at night is called land breeze.
10. Radiation: The process of transmission of heat without any medium
is called radiation. We receive heat from the sun by the
way of radiation.
11. Thermos flask: The thermos flask is a special kind of bottle which
prevents all the possible ways of flow of heat to and
from the bottle.
12. Temperature: The degree of hotness or coldness of a substance is
called temperature.
13. Thermometer: An instrument that measures temperature is called a
thermometer.
14. Thermometric liquid: A liquid which is used in the bulb of a liquid thermometer
is called the thermometric liquid.
15. Principle of a thermometer: A thermometer is based on the principle that “matter
expands when its temperature increases and contracts
when its temperature decreases”.
6.1 Introduction
Heat is a form of energy which gives us sensation of warmth. A body is made up of a
large number of particles called molecules. The arrangements of molecules in solids,
liquids and gases are different. Such molecules may possess translational, vibrational
and rotational motions. Due to the motion of molecules in a substance, they have
some kinetic energy which is called heat. Thus, the sum of kinetic energy of all the
molecules in a substance is called heat. Thus, everybody contains some amount of
heat. The greater the kinetic energy of the molecules of a body, the greater is the heat.
The average kinetic energy of the molecules in a substance determines its degree of
hotness or coldness, i.e. temperature.
When we rub or strike a substance, the kinetic Calorimeter Thermometer
energy of the molecules increases. This results in the
increase in heat as well as temperature. In this unit
we will learn about the flow of heat, applications
of heat, different modes of transmission of heat,
temperature and measurement of temperature.
Modern Concept Science and Environment - 7 73
FACT WITH REASON
A match stick produces flame when its head is rubbed sharply against the side of a matchbox.
When the head of a match stick is rubbed sharply against the rough surface on the side of a
matchbox, the kinetic energy of the molecules on the surface increases due to friction. The surface
gets heated and produces a tiny spark. This spark ignites the chemical substance on the head of
the match stick. Thus a match stick produces flame when its head is rubbed sharply against the
side of a matchbox.
Sources of Heat
The substance or medium which gives us heat is called a source of heat. Some
important sources of heat are sun, bio-fuel, fossil fuel and electricity.
Need of Heat
Why do we need heat? In how many ways do we use heat in our daily life? One can
answer these questions by looking at our everyday surroundings. Heat is required for
a variety of reasons. Some of them are mentioned below.
1. Heat is an essential form of energy for all plants and animals to perform various
metabolic activities.
2. We require heat to cook food, to light a lamp, to iron clothes or to take a hot
water bath.
3. Goldsmiths use heat from a flame to design an ornament.
4. In most of the industries and factories, heat is the main source of energy to run them.
Effects of Heat
When an object is heated various changes take place in the object.
a) Heat changes state of matter: On heating, a substance changes its state from a
solid to a liquid or from a liquid to a gas.
b) Heat changes temperature of a body: On heating a substance, the average kinetic
energy of its molecules increases and this causes an increase in its temperature.
c) Thermal Expansion: Matters expand on heating and contract on cooling.
FACT WITH REASON
A metal becomes hot when it is struck with a hammer. Why?
When a metal surface is struck with a hammer, it causes the molecules in the metal to vibrate
faster. Such vibrations increase the kinetic energy of the molecules, that is heat.
Units of Heat MEMORY PLUS
Calorie and joule are the two commonly used units 1 calorie heat is equal to 4.2 joules.
of heat. For example, energy of food is specified in
kilocalories (kcal).
74 Heat
6.2 Transfer of Heat
The process of transfer of heat from one place to another is called the transmission
of heat. Heat always flows from a body at higher temperature to a body at lower
temperature.
ACTIVITY 1
To show heat flows from a body at a higher temperature to one at a lower temperature.
1. Take some hot water in a tumbler.
2. Dip a metal spoon in that water for some time. Notice the time taken for the spoon to
become hot.
3. Leave the tumbler for 20 minutes. Do you feel the water cooling down?
It becomes uncomfortable to hold the hot spoon due to the heat transferred from the hot
water in the tumbler to the spoon. This is due to the flow of heat from a body at a higher
temperature to one at a lower temperature.
Have you ever put some ice on your palm? Also, what do you feel when your finger is
in a very warm water even for one second? In such cases, our sense of touch estimates
the degree of coldness and hotness of a body on the basis of the flow of heat. We feel
cold if the heat flows out of our body. Similarly, We feel hot if the heat flows into our
body. If the heat energy flows in or out of the body very rapidly, we have a very hot
or a very cold sensation accordingly. Thus,
a) Heat can be transferred from one body to another body.
b) Heat flows from a body at a higher temperature to one at a lower temperature.
FACT WITH REASON
Why does heat flow from a hotter object to a colder objects?
Heat flows from a hotter object to a colder object in order to balance the temperature
between them. Heat continues flowing until both objects have equal temperature.
Why do we feel cool when we put our legs in cold water?
We feel cool while putting our legs in cold water because our legs lose heat continuously.
6.3 Modes of Transmission of Heat
We feel warm while sitting in the sunlight. A cup of tea left open for some time gets
cold. A steel spoon becomes hot while using it to stir the food being cooked on a stove.
These all happen because of the heat transfer taking place through a medium. Heat
transfers through solids, liquids and gases. The mode of transmission of heat depends
on a medium.
Modern Concept Science and Environment - 7 75
Heat can be transferred from one place to another Convection Conduction
by three different ways. They are conduction,
convection and radiation.
Mode of Transmission Medium
Conduction Solid
Convection Liquid and gas Radiation
Radiation Vacuum
Conduction Transmission of heat
ACTIVITY 2
To show the process of conduction.
1. Take an iron rod of about 25cm length.
2. Fix thumb pins along the length of the rod, using wax, Direction in which heat travels
as shown in the figure.
3. Fix one end of the rod on a stand and heat another end
of the rod as shown in the given figure. Do you observe
that the pins start to fall off after some time?
As the rod gets heated, the heat gets conducted through the
rod to the wax. The wax melts and the thumb pins fall off. The thumb pin closest to the end
of the rod being heated falls first. The farthest thumb pin falls at last.
Heat transmits along the length of the rod from the hot end to the cold end without
actual movement of the molecules in it. The process of transfer of heat in solids from
one molecule to the neighboring molecule without their actual movement is called
conduction.
Mechanism of Conduction
When a metallic rod is heated then its molecules start vibrating faster. These molecules
at the heated end hit the nearby molecules and make them vibrate too. During this
process they give heat to the nearby molecules. These molecules again give heat to
the next closest molecules. In this way the heat gets conducted from the hot end to the
cold end.
Heat Heat Heat
76 Heat
FACT WITH REASON
Why is conduction common in metals?
Conduction is common in metal because molecules of metal are closely packed and cannot move easily.
A steel cup becomes hot on pouring hot water in it. Why?
Molecules of steel cup transfer heat to the neighboring molecules when hot water is added in it. In
this way heat transfers throughout the steel cup and it becomes hot.
Conditions for Conduction
i) The temperature of the objects should not be the same.
ii) Two objects at different temperatures must be in contact.
Conductors and Insulators of Heat
ACTIVITY 3
To show that some materials are good conductor of heat and some are bad conductor
of heat.
1. Take a steel spoon, a glass rod, a plastic scale, a wooden stick, etc. and dip one end
of each of them in hot water.
2. Then touch the other end of each article with your finger. Do you feel some of
them get heated and some do not get heated?
Conductors
The materials which allow heat to flow through them are called conductors of heat. The
rate of transfer of heat energy is different in different conductors. On the basis of the rate
of transfer of heat, conductors are classified as good conductors and bad conductors.
i) Good Conductors of Heat
The materials which allow heat to flow through them easily are called good
conductors of heat. For example, metals like iron, copper, silver and aluminum are
good conductors of heat. Most metals are good conductors of heat. Cooking utensils
are made of good conductors of heat.
FACT WITH REASON
Iron is good conductor of heat. Why?
Iron is good conductor of heat because iron allows heat to flow through it easily when comes in
contact with a hot body.
A chair made of metal is very cold to touch in winter. Why?
Metals are good conductor of heat. In winter, a metal chair is at lower temperature. When we
touch it, the heat from our body quickly transmits into it. So, a chair made of metal is very cold to
touch in winter.
Modern Concept Science and Environment - 7 77
ii) Bad Conductors of Heat MEMORY PLUS
Some substances conduct little or no heat. They are An ice cube will soon melt if we hold
called bad conductors of heat. Water, glass, wood, it in our hand. This is because, the
rubber, clay and all gases including air, are bad heat is being conducted from our
conductors of heat. hand into the ice cube.
Insulators
Those materials which do not allow heat to flow through them are called insulators of
heat. For example, ebonite, asbestos, etc. are insulators. Handles of cooking utensils
are made of insulators.
FACT WITH REASON
Handles of cooking utensils are made of hard plastics. Why?
Hard plastics do not allow heat to flow through them as they are insulators. Therefore,
handles of cooking utensils are made of insulators.
Applications of Thermal Conductivity
a) Quilts are filled with fluffy cotton : Air enters inside the pores of cotton. Both
cotton and air are bad conductors of heat. So the cotton does not allow the heat
to flow from our body to the surrounding. It provides a good heat insulation. As
a result, our body remains warm.
ii) Cooking utensils are made of metals : Metals are good conductors of heat.
Heat can get conducted easily through the base of the utensils.
iii) Handles of cooking utensils are made from insulators : Ebonite insulators
on the handles of cooking utensils do not allow heat to get conducted from the
utensils to our hand.
iv) Very hot water poured in a thick glass tumbler cracks it : Glass is a bad
conductor of heat. When very hot water is poured in a glass tumbler, the inner
surface expands due to the conduction of heat. This heat does not reach the
outer surface quickly. Hence, the inner surface expands but the outer surface
does not and the glass cracks.
v) The walls of cold storage rooms are made thicker : Bricks are bad conductors
of heat. Thick walls of the brick reduce the flow of heat from the surrounding to
the rooms.
vi) To prevent ice from melting, it is covered with jute and sawdust : Saw dust
and jute are bad conductors of heat. A large amount of air is trapped in them.
This trapped air acts as a layer of insulation. It does not let the ice to gain heat
from the surroundings.
78 Heat
vii) Woolen clothes are warm : Wool and air are bad conductors of heat. The fine
pores in woolen clothes are filled with air. Therefore, the woolen clothes check
the flow of heat from our body to the surroundings and keep our body warm.
viii) In cold countries, windows have two panes or three panes with a thin layer
of air in between : Air in between two panes of windows is a bad conductor. It
checks the conduction of heat from the room to the surroundings.
FACT WITH REASON
Birds sometimes fluff out their feathers on a cold winter morning. Why?
Air is a poor conductor of heat. Birds trap air between feathers by fluffing them up. The
trapped air provides insulation. It does not let the heat to go out of their body. So, birds
sometimes fluff out their feathers on a cold winter morning.
Convection
In conduction, the heated molecules do not leave their mean position. But in convention,
there is an actual movement of the heated molecules. The process of transfer of heat
by the actual movement of molecules in liquids and gases is called convection. For
example, hot lighter air moves up and cold air settles down.
ACTIVITY 4
1. Take a rectangular glass box with two openings ‘A’ and ‘B’, as shown in the given figure.
2. A lighted candle is placed below the opening ‘A’. A B
3. Take a source of smoke like joss stick and place it above the Hot air Cold air
rising up falling
opening ‘B’. chimney down
chimney
Do you observe the smoke moving in through ‘B’ and moving
out through ‘A’? This is due to convection. The lighted candle
warms the air in contact with its flame. Which becomes light
and goes out of the box through ‘A’. The place of hot light air is
occupied by the cold smoke air. This causes the smoke to move in through the opening
‘B’ and move out through the opening ‘A’.
a) Mechanism of Convection
Consider water being heated in a pot as shown in the
given figure. The density of water at the bottom of
the pot decreases when it gets heated first. These hot
molecules with high kinetic energy rise upward while
the cold molecules come down to take their place. This
causes a flow of water molecules in the pot and the flow is known as the convection
current. This convection current transfers heat to the entire mass of water.
Modern Concept Science and Environment - 7 79
FACT WITH REASON
Iron is good conductor of heat. Why?
Iron is good conductor of heat because iron allows heat to flow through it easily when comes in
contact with a hot body.
A chair made of metal is very cold to touch in winter. Why?
Metals are good conductor of heat. In winter, a metal chair is at lower temperature. When we
touch it, the heat from our body quickly transmits into it. So, a chair made of metal is very cold to
touch in winter.
b) Cause of Wind
During the day time, air around the earth gets heated by the solar radiation. Hot
lighter air moves up and colder air occupies the space of hot air. This phenomenon
causes the wind.
Applications of Convection
a) Ventilators in a Room
Ventilators are made on ceiling or near the ceiling on
the wall of a room. A ventilator in a room maintains a
fresh flow of the air in the room. Fresh air is important
for us to breathe. The air that we breathe out is
warmer and hence lighter. The fresh air enters into
our room through the window and ventilation and
the warm air goes out through the ventilator. This Ventilation
sets up a convection current and maintains a continuous flow of fresh air in the room.
b) Chimneys
In factories, chimneys are fitted to remove undesirable smoke and fumes. The light
smoke, fumes and hot gases rise up through the chimneys.
c) Installation of Air-conditioners
For effective cooling, air-conditioners are at a height above the level of the windows.
The cool air from the air-conditioner sinks, whereas the warm air of the room rises.
d) Installation of Room Heater Warm air
For effective heating, room heaters are at the ground rising
level. Air molecules in a room, when they come in contact
with the heater, get heated and gain the kinetic energy.
Such hot and light molecules rise upward. The cool air Cold air falling
molecules occupy the space of hot molecules. This sets
up convection currents in the room and heat transfers to Cold door Hot radiator
different parts of the room. Room heater
80 Heat
e) Installation of the Freezer
In a refrigerator, the freezer is always made at the top. The cold Refregirator
air which sinks down from the freezer cools the things kept at the
lower part of the refrigerator. At the same time, the heat released
in the lower part of the refrigerator makes the air warm and it rises
up to the freezer. Thus, the freezer at the top sets up convection
current and keeps all parts of the refrigerator cool.
f) Land Breeze and Sea Breeze
Land gets heated faster as well as loses the heat faster. Water, however, has just the
opposite characteristic. During the day, the land becomes hot and air around it also
becomes hot. Such hot and light air rises up. This place is occupied by the cold air
above the sea. Thus, during the day, cold air above the sea blows towards the land
which is called a sea breeze.
Warm air warm air
Cold sea breeze Cool
land breeze
day land warmer Sea cooler night land cooler Sea warmer
Sea breeze Land breeze
During the night, the land becomes cold and air around it also becomes cold. But the
air above the sea remains hot. The hot and light air rises up. This place is occupied
by the cold air above the land. Thus, during the night, cold air above the land blows
towards the sea which is called a land breeze.
FACT WITH REASON
Land breeze occurs at night. Why?
Land breeze occurs at night because land is colder than ocean during nights and air always blows
from colder place to hotter place.
Modern Concept Science and Environment - 7 81
Radiation
In winter, we feel warm when we sit in the sunlight. How Radiation
does the heat travel from the sun to our body? Such a
transmission is neither conduction nor convection. There
is nothing in the space between the sun and out of the
earth’s atmosphere. The heat from the sun travels through
a vacuum in the form of rays called radiation. Thus, the
process of transmission of heat without any medium is
called radiation. Hence we receive heat from the sun by the
way of radiation.
FACT WITH REASON
Even without any medium, heat from the sun reaches to the earth. How?
Heat from the sun reaches us even without a medium because sunlight is an electromagnetic
wave. It does not need medium to travel.
Applications of Radiation
a) Electric room heater
The curved polished surface behind the rod-like heating element reflects the radiation
falling on it. We feel the warmth even if we stand a few feet away from the heater.
b) Choice of clothes
White clothes absorb less heat radiation than clothes of other colors. So, we wear
white or light-color clothes in summer. On the other hand, we wear black and dark-
color clothes in winter. Such clothes absorb more heat and keep us warm.
c) Solar heaters and solar cookers MEMORY PLUS
Heat from the sun is used to heat water in a solar Black bodies are good absorber and
water heater. The inner surface of solar cooker is good radiator of heat.
polished with black. The black surface absorbs the
heat. Shining bodies are poor absorber
and poor radiator of heat.
d) Radiators of cars and air conditioners:
The black coating on radiators of cars and air-conditioners absorbs maximum heat
radiation and produces the cooling effect.
FACT WITH REASON
Why do people wear white clothes in summer season?
People wear white clothes in summer season because white clothes reflect heat and keep us cool.
82 Heat
Conduction, Convection and Radiation
Parameter Conduction Convection Radiation
Medium Needs a medium Needs a medium Does not need a
medium
Particles Particles in the Particles of the
movement medium do not leave medium leave their Particles of the
their mean position. mean position. medium are not
Speed involved
It is a slow process. It is faster than
conduction It takes place at a
speed of light.
6.4 Thermos Flask
Thermos flask is a special kind of bottle which Thermos without Thermos with
prevents all possible ways for the flow of heat to reflective lining reflective lining
and from the bottle. In a thermos flask, hot liquids
like tea, milk, etc. remain hot and cold items like Conduction Conduction
ice cubes, cold water, etc. remain cold for a long Convection Convection
period of time. A thermos flask is also called a
vacuum flask. Radiation Radiation
Reflective
lining
Construction of a Thermos Flask Thermos
A thermos flask consists of a double-walled glass MEMORY PLUS
bottle. The air between the two layers is evacuated
and the two layers are sealed. This double walled Sir James Dewar first invented a
bottle is fixed inside a plastic or metallic casing for thermos flask in 1892. That is why
its protection. Walls of the bottle are polished to look a thermos flask is also called Dewar
flask.
silvery. These shining polished surfaces reduce the heat loss by radiation. The glass
walls are bad conductors of heat. Hence the flow of heat is reduced by conduction.
Also the vacuum in between glass walls reduces the heat losses due to conduction and
convection.
Body Parts of A Reduced Mode of Heat Reason
Thermos Flask Exchange
Plastic cap Convection, evaporation It prevents hot air from leaving the
thermos flask
Double walled Conduction, convection There is no matter to exchange heat
bottle with vacuum in a vacuum.
Silvery walls Radiation These walls reflect the heat radiation
passing through the vacuum
Modern Concept Science and Environment - 7 83
Advantages of a Thermos Flask
In a thermos flask, all ways of heat loss viz. MEMORY PLUS
conduction, convection and radiation are highly
reduced. So a thermos flask keeps the hot object hot Vacuum flasks are often used as
and the cold object cold inside it for a long time. insulated shipping containers.
FACT WITH REASON
Why do ice box have double wall?
Ice box have double wall because air trapped inside the double wall, acts as insulator and stops
heat from reaching to ice. Ice will not melt sooner.
6.5 Temperature
In our daily life, when we touch a bottle of cold drink, a milk pouch, etc. which are
kept in a refrigerator, we feel cold. On the other hand when we touch a cooking utensil
on fire, boiled water, etc. we feel hot. The degree of hotness or coldness of a substance
is called its temperature.
The temperature of a body is an indicator of the average kinetic energy of the molecules
of the body. The greater the average kinetic energy of the molecules of a body, the
greater is the temperature of the body.
FACT WITH REASON
We feel cold on touching ice and hot on touching heated objects
i. When we touch a cold body, such as ice, heat transfers from our hand to the ice and we
feel cold.
ii. When we touch a hot body, such as a hot potato, heat transfers from the hot potato into
our hand and we feel hot.
Differences between Heat and Temperature
Heat Temperature
1. Heat is a form of energy which gives 1. Temperature is the degree of hotness
a sensation of warmth. and coldness of a body.
2. Heat is measured by a calorimeter. 2. Temperature is measured by a
thermometer.
3. It measures the total kinetic energy 3. It measures the average kinetic
of all the molecules in a substance. energy of all the molecules in a
substance.
4. Heat is the cause of change in 4. Change in temperature is the effect
temperature. of heat.
84 Heat
6.6 Measurement of Temperature
The property of expansion of matter is used to MEMORY PLUS
measure temperature. It is measured generally
in the three different scales with the help of The normal temperature of human
thermometers. body is 37oC or 98.6°F.
Thermometer
A thermometer is an instrument that measures the temperature. It is based on the
property of a matter that changes with temperature. If a gas is used in a thermometer,
it is called a gas thermometer. There are solid and liquid thermometers too. The
commonly used thermometer is the liquid thermometer.
Thermometric Liquid
A liquid which is used in the bulb of liquid thermometers is called a thermometric
liquid. Usually mercury or colored alcohol are used as the thermometric liquids.
The properties of a good thermometric liquids are:
i) It should be a good conductor of heat.
ii) It should be opaque for clear visibility.
iii) It should expand uniformly throughout the measuring range.
iv) It should be in the liquid state throughout the measuring range.
FACT WITH REASON
Why is mercury used as thermometric liquid?
Mercury is used as thermometric liquid because it is good conductor of heat, opaque and shiny.
Principle of Thermometer
A thermometer is based on the principle that “matter expands when its temperature
increases and contracts when its temperature decreases”.
Thermometer
Construction of a Thermometer
a) Capillary Tube: The thermometer consists of a very fine glass tube having a
very small bore, called a capillary tube.
Modern Concept Science and Environment - 7 85
b) Stem: The capillary tube is protected by a thick glass tube called the stem.
c) Bulb: At one end of the capillary tube, a very thin walled glass is provided,
called a bulb. It is filled with a thermometric liquid.
d) Standard Points: Steam point (i.e. upper fixed point) and ice point (i.e.
lower fixed point) : Two standard points are fixed on the stem of a thermometer
to make a thermometric scale.
a) Lower Fixed Point: The melting point of pure ice at a pressure of 760
mmHg is taken as the lower fixed point. Its value is 00C.
b) Upper fixed point: The boiling point of pure water at a pressure of 760
mmHg is taken as the upper fixed point. Its value is 1000C.
e) Calibration: In between the upper fixed point MEMORY PLUS
and lower fixed point, a number of equal
divisions are made. This process is called Stem of a clinical thermometer is of
calibration. The markings on the stem are prismatic shape.
called graduations or degrees.
FACT WITH REASON
The bulb of a thermometer is made thin wall. Why?
The bulb of a thermometer consists a thermometric liquid. The thin wall of the bulb helps flow of
heat from outside to the thermometric liquid inside.
Measurement of Temperature by a Thermometer
One of the most familiar thermometer is the mercury thermometer. Pure mercury
is filled in the bulb of this thermometer. When the bulb comes in contact with a hot
surface, the mercury in the bulb expands and its volume increases. As the temperature
increases, the mercury is forced to rise in the capillary tube. The rise in the level of
mercury depends on the degree of hotness. Temperature is measured by reading the
rise in the level of mercury column in degrees.
A clinical thermometer is used to measure our body temperature. It is also called the
doctor’s thermometer. In laboratories, there is a laboratory thermometer to measure
the temperature of different substances.
Advantages and Disadvantages of a Mercury Thermometer
Advantages of Mercury as a Thermometric Liquid
i) Mercury is a good conductor of heat. It needs very little heat to expand.
86 Heat
ii) It does not wet the glass. Thus, it helps in recording temperature with better
accuracy.
iii) Expansion of mercury is uniform over a wide range of temperatures.
iv) It is opaque and silvery in color. Thus, its level of rise and fall can be clearly seen
inside the capillary tube.
v) The boiling temperature of mercury is 357 0C. So a temperature as high as 3570C
can be recorded with the help of a mercury thermometer.
FACT WITH REASON
Mercury thermometer cannot be used in cold places. Why?
Mercury thermometer cannot be used in cold places because it freezes at -39°C.
Disadvantages of Mercury as a Thermometric Liquid
i) Freezing temperature of mercury is -39 0C. A mercury thermometer can
record a low temperature only up to -39 0C. So, a mercury thermometer
cannot measure a very low temperature, such as the temperature in the Arctic
and Antarctic regions.
ii) Mercury is a poisonous substance. It is risky if the thermometer tube is broken.
Advantages and Disadvantages of an Alcohol Thermometer
Advantages of Alcohol as a Thermometric Liquid MEMORY PLUS
i) The melting point of alcohol is -117°C. So an
Since 2001, most of the states of America
alcohol thermometer can record a temperature banned mercury thermometers.
as low as -117°C. An alcohol thermometer can record the temperature in Arctic
and Antarctic regions.
ii) Alcohol expands more than mercury for the same rise in temperature. Thus, an
alcohol thermometer can measure temperature more accurately than mercury
thermometer.
Disadvantages of an Alcohol Thermometer MEMORY PLUS
i) Alcohol needs a bright color for its use in a
Alcohol expands seven times more
thermometer. than that of mercury.
ii) The boiling temperature of alcohol is 78°C. So an alcohol thermometer cannot
measure the temperature of boiling water.
Modern Concept Science and Environment - 7 87
FACT WITH REASON
Alcohol thermometer cannot be used to measure temperature of boiling water. Why?
Alcohol thermometer cannot be used to measure temperature of boiling water because it
boils before water at 78°C
STEPS EXERCISE
STEP 1
1. Tick () the correct statement and cross () the incorrect one.
a) Heat is a form of energy.
b) Heat always transfers from a body at a low temperature to a body at a high
temperature.
c) Heat transmits by conduction in a solid substance.
d) Freezing temperature of mercury is -39 °C.
e) Heat transmits by radiation in vacuum.
f) A mercury thermometer can measure a very low temperature.
2. Fill in the blanks with appropriate words
a) Heat transmits by convection in …… and …….
b) The heat energy from the sun reaches the earth due to the process of ……. .
c) Mercury is the metal which gets heated by the process of …….
d) …… or …… is used as a thermometric liquid.
e) Mercury boils at …… °C.
f) The instrument which measures temperature is called …… .
g) …… thermometer can measure a very high temperature.
STEP 2
3. Answer in one word.
a) What is the name of the instrument used in measuring the temperature?
b) In how many ways does heat transmit?
c) What is the term for the degree of hotness or coldness?
d) Which thermometer is used to measure body temperature?
e) What happens to the volume of a liquid on heating it?
4. Differentiate between
a) Heat and temperature
88 Heat
b) Conduction and convection
c) Alcohol thermometer and mercury thermometer
5. Give reason:
a) On touching an object we feel cold in winter season and hot in summer season.
b) Handle of a metal pot becomes hot during cooking.
c) Air blows from one place to another.
d) Birds sometimes fluff out their feathers on a cold winter morning.
e) Convection is not possible in solids.
f) Ice box has a double wall.
6. Answer the questions with the help of the given figures. Direction in which heat travels
a) Which mode of transmission of heat is shown in
the given figure?
STEP 3
7. Answer the following
a) What are the sources of heat? Give examples.
b) What is transmission of heat? What are its modes?
c) Define conduction, convection and radiation with examples.
d) Explain the mechanism of convection of heat.
e) How do convection currents in air help in ventilation?
f) Explain the construction of a thermos flask with its use.
g) Explain how heat loss due to conduction, convection and radiation are
minimized in a thermos flask?
h) What is temperature?
i) What is a thermometer? On which principle is it constructed?
j) What is a thermometric liquid?
k) Explain the construction of a thermometer.
l) How is the temperature measured by using a thermometer?
m) List the advantages and disadvantages of a mercury thermometer.
n) List the advantages and disadvantages of an alcohol thermometer.
8. Draw the diagrams
a) to show the internal structure of a thermos flask.
b) to show the land breeze and sea breeze.
UNIT Estimated teaching periods TheorMyoderPnraCcotniccaelpt Science and Environment - 7 89
41
7
Light
Syllabus issued by CDC Sun
Introduction to light
Sources of light
Reflection of light
Types of reflection of light
Laws of reflection of light
Reflection of light from a plane mirror
Periscope and Kaleidoscope
LEARNING OBJECTIVES
At the end of this unit, students will be able to:
define reflection of light and explain its types.
state and demonstrate the laws of reflection of light.
construct some equipment like Periscope and Kaleidoscope and explain their
working mechanism.
Key terms and terminologies of the unit
1. Light: Light is a form of energy which gives sensation of vision to us.
2. Medium: The substance through which light propagates or tends to
propagate is called a medium.
3. Ray of light: The straight path along which the light travels through a medium
is known as a ray.
4. Beam of light: A group of light rays originating from a source is called a beam of
light.
5. Reflection of light: The bouncing of light on the same medium after sticking a surface
is called reflection of light.
6. Regular reflection: If a parallel beam of light falls on a reflecting surface and gets reflected
as a parallel beam, the reflection is known as regular reflection.
7. Irregular reflection: If a parallel beam of light falls on a reflecting surface but it gets
reflected not as a parallel beam, then the reflection is known as
an irregular reflection.
90 Light
8. Real image: An image which can be obtained on a screen is called a real image.
9. Virtual image: An image which cannot be obtained on a screen is called a virtual image.
10. Mirror: A smooth polished surface from which a regular reflection can take
place is called a mirror.
11. Periscope: A periscope is an optical device that allows us to see over the walls.
12. Kaleidoscope: A kaleidoscope is a simple instrument based on the laws of reflection.
7.1 Introduction
Objects are not visible in a dark room. However, if MEMORY PLUS
we switch on a bulb, the objects become visible.
When light falls on the objects, it bounces off their Light travels with a speed of
surfaces. We can see things around us because light 300,000,000 m/s in air or vacuum.
enters our eyes and forms an image of these things on our retina. Thus, light is a form
of energy which gives sensation of vision to us. Objects are not visible without light.
The study of light and vision is called optics.
7.2 Sources of Light
Those bodies which emit light are called sources of light. For example, sun, electric
bulb, lamp, etc. Sources of light are of two types.
They are self-luminous and non-luminous sources.
Light that enters into our eyes may have come
directly from a self-luminous objects or it may have
come as reflection from a non-luminous objects. Sources of light
Self-luminous sources of light emit their own light.
For example, sun, star, firefly, etc. On the other hand, MEMORY PLUS
objects like moon, soil, rock, plastic, glass, etc. do not Moon is also a non-luminous body
as it does not have its own light.
emit light. They are called non-luminous objects.
FACT WITH REASON
Sun is called a luminous body. Why?
There is a release of huge amount of heat and light from the sun by thermonuclear fusion.
So, the sun is a luminous body.
Optical Medium
The substance through which light propagates or tends to propagate is called a
medium. For example, glass, water, air etc. Light travels at a high speed in the form of
a wave. In vacuum, it travels at the rate of 3 hundred thousand kilometers per second.
The speed of light changes with the change in the medium through which it travels.
Modern Concept Science and Environment - 7 91
Ray of Light
The straight path along which the light travels through a A ray of light
medium is known as a ray. It is represented by an arrow head
on a straight line. The arrow head of a ray gives its direction. Rays are produced when
light shines through a small hole.
Beam of Light
A group of light rays originating from a source is called a beam of light. A beam of
light is produced when light shines through a larger hole. There are three types of
light beam. They are convergent beam, divergent beam and parallel beam.
Parallel beam of light Convergent beam of light Divergent beam of light
7.3 Reflection of Light
When a beam of light falls on a surface, then there occurs three possibilities.
1. A part of light may be absorbed.
2. A part of light may return to the same medium, giving rise to a phenomenon
called reflection of light.
3. A part of light may get transmitted through the medium, giving rise to a
phenomenon of refraction of light.
FACT WITH REASON
Why do mirrors reflect light?
Mirrors reflect light because they have very smooth surface.
When the rays of light fall on a smooth surface, the rays of light return to the same
medium. This phenomenon is called reflection of light. Thus, the bouncing of light on
the same medium after sticking a surface is called reflection of light.
General Terms Used in Reflection of Light
a) Incident ray : The light ray which falls upon a reflecting surface is called an
incident ray. In the given figure AB is an incident ray.
b) Point of incidence : The point on the reflecting surface, where an incident ray
strikes is called the point of incidence. In the given figure, ‘B’ is the point of
incidence.
92 Light
c) Reflected ray : The light ray which bounces away from the reflecting surface is
called a reflected ray. BC is the reflected ray Incident ray
in the given figure. A Normal
D Reflected ray
d) Normal : A perpendicular line drawn at the Incident Reflected C
point of incidence on a reflecting surface is angle
angle i=r
i r
called the normal. In the given figure, BD is
the normal line. B
e) Angle of incidence : The angle made by
Mirror
the incident ray with the normal is called
the angle of incidence. ∠ABD = i is the angle Reflection of light
of incidence in the given figure. MEMORY PLUS
f) Angle of reflection : The angle made by the The things we see in a day light is
reflected ray with the normal is called an angle due to reflection of light.
of reflection. ∠CBD = r is the angle of reflection in
the given figure.
Laws of Reflection
1. The incident ray, the reflected ray and the normal
at the point of incidence all lie in the same plane.
2. The angle of incidence is equal to the angle of
reflection.
ACTIVITY 1
1. Take a wooden drawing board and fix a sheet of white paper on it.
2. In the middle of the paper, draw a fine straight line XY M M’
and mark a point B in the middle of the line. Draw a
perpendicular BN and a line AB by making an angle with Mirror
BN. XB Y Wooden
3. Place a mirror MM’ along the line XY such that the
QR board
reflecting surface faces the line AB and fix it with the help PS
a mirror stand. AN
4. Fix two common pins P and Q on the straight line AB.
5. Look for the image of pins P and Q in the mirror. Once the images are located, fix
two more pins R and S, such that the pins P, Q, R and S look to fall on the same
straight line.
6. Remove the mirror and pins and join the marks of R and S.
7. Measure ∠ PBN and ∠SBN. Do you find them equal?
8. This activity proves that the angle of incidence is equal to the angle of reflection.
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